Cancer treatments, in general, have a higher rate of success if the cancer is diagnosed early and treatment is started earlier in the disease process. The relationship between improved prognosis and stage of disease at diagnosis hold across all forms of cancer for the most part. Therefore, there is an important need to develop early assays of general tumorigenesis through marker assays that measure general tumorigenesis without regard to the tissue source or cell type that is the source of a primary tumor. Moreover, there is a need to address distinct genetic alteration patterns that can serve as a platform associated with general tumorigenesis for early detection and prognostic monitoring of many forms of cancer.
Importance of DNA Methylation
DNA methylation is a mechanism for changing the base sequence of DNA without altering its coding function. DNA methylation is a heritable, reversible and epigenetic change. Yet, DNA methylation has the potential to alter gene expression, which has profound developmental and genetic consequences. The methylation reaction involves flipping a target cytosine out of an intact double helix to allow the transfer of a methyl group from S-adenosylmethionine in a cleft of the enzyme DNA (cystosine-5)-methyltransferase (Klimasauskas et al., Cell 76:357-369, 1994) to form 5-methylcytosine (5-mCyt). This enzymatic conversion is the only epigenetic modification of DNA known to exist in vertebrates and is essential for normal embryonic development (Bird, Cell 70:5-8, 1992; Laird and Jaenisch, Human Mol. Genet. 3:1487-1495, 1994; and Bestor and Jaenisch, Cell 69:915-926, 1992). The presence of 5-mCyt at CpG dinucleotides has resulted in a 5-fold depletion of this sequence in the genome during vertebrate evolution, presumably due to spontaneous deamination of 5-mCyt to T (Schoreret et al., Proc. Natl. Acad. Sci. USA 89:957-961, 1992). Those areas of the genome that do not show such suppression are referred to as "CpG islands" (Bird, Nature 321:209-213, 1986; and Gardiner-Garden et al., J. Mol. Biol. 196:261-282, 1987). These CpG island regions comprise about 1% of vertebrate genomes and also account for about 15% of the total number of CpG dinucleotides (Bird, Infra.). CpG islands are typically between 0.2 to about 1 kb in length and are located upstream of many housekeeping and tissue-specific genes, but may also extend into gene coding regions. Therefore, it is the methylation of cytosine residues within CpG islands in somatic tissues, which is believed to affect gene function by altering transcription (Cedar, Cell 53:3-4, 1988).
Methylation of cytosine residues contained within CpG islands of certain genes has been inversely correlated with gene activity. This could lead to decreased gene expression by a variety of mechanisms including, for example, disruption of local chromatin structure, inhibition of transcription factor-DNA binding, or by recruitment of proteins which interact specifically with methylated sequences indirectly preventing transcription factor binding. In other words, there are several theories as to how methylation affects mRNA transcription and gene expression, but the exact mechanism of action is not well understood. Some studies have demonstrated an inverse correlation between methylation of CpG islands and gene expression, however, most CpG islands on autosomal genes remain unmethylated in the germline and methylation of these islands is usually independent of gene expression. Tissue-specific genes are usually unmethylated and the receptive target organs but are methylated in the germline and in non-expressing adult tissues. CpG islands of constitutively-expressed housekeeping genes are normally unmethylated in the germline and in somatic tissues.
Abnormal methylation of CpG islands associated with tumor suppressor genes may also cause decreased gene expression. Increased methylation of such regions may lead to progressive reduction of normal gene expression resulting in the selection of a population of cells having a selective growth advantage (i.e., a malignancy).
It is considered that altered DNA methylation patterns, particularly methylation of cytosine residues, cause genome instability and are mutagenic. This, presumably, has led to an 80% suppression of a CpG methyl acceptor site in eukaryotic organisms, which methylate their genomes. Cytosine methylation further contributes to generation of polymorphism and germ-line mutations and to transition mutations that inactivate tumor-suppressor genes (Jones, Cancer Res. 56:2463-2467, 1996). Methylation is also required for embryonic development of mammals (Bestor and Jaenisch, Cell 69:915-926, 1992). It appears that that the methylation of CpG-rich promoter regions may be blocking transcriptional activity. Therefore, there is a probability that alterations of methylation are an important epigenetic criteria and can play a role in carcinogenesis in general due to its function of regulating gene expression. Ushijima et al. (Proc. Natl. Acad. Sci. USA 94:2284-2289, 1997) characterized and cloned DNA fragments that show methylation changes during murine hepatocarcinogenesis. Data from a group of studies of altered methylation sites in cancer cells show that it is not simply the overall levels of DNA methylation that are altered in cancer, but changes in the distribution of methyl groups.
These studies suggest that methylation, at CpG-rich sequences known as CpG islands, provide an alternative pathway for the inactivation of tumor suppressors, despite the fact that the supporting studies have analyzed only a few restriction enzyme sites without much knowledge as to their relevance to gene control. These reports suggest that methylation of CpG oligonucleotides in the promoters of tumor suppressor genes can lead to their inactivation. Other studies provide data that suggest that alterations in the normal methylation process are associated with genomic instability (Lengauer et al. Proc. Natl. Acad. Sci. USA 94:2545-2550, 1997). Such abnormal epigenetic changes may be found in many types of cancer and can, therefore, serve as potential markets for oncogenic transformation, provided that there is a reliable means for rapidly determining such epigenetic changes. The present invention was made to provide such a universal means for determining abnormal epigenetic changes and address this need in the art.
Methods to Determine DNA Methylation
There is a variety of genome scanning methods that have been used to identify altered methylation sites in cancer cells. For example, one method involves restriction landmark genomic scanning (Kawai et al., Mol. Cell. Biol. 14:7421-7427, 1994), and another example involves methylation-sensitive arbitrarily primed PCR (Gonzalgo et al., Cancer Res. 57:594-599, 1997). Changes in methylation patterns at specific CpG sites have been monitored by digestion of genomic DNA with methylation-sensitive restriction enzymes followed by Southern analysis of the regions of interest (digestion-Southern method). The digestion-Southern method is a straightforward method but it has inherent disadvantages in that it requires a large amount of DNA (at least or greater than 5 .mu.g) and has a limited scope for analysis of CpG sites (as determined by the presence of recognition sites for methylation-sensitive restriction enzymes). Another method for analyzing changes in methylation patterns involves a PCR-based process that involves digestion of genomic DNA with methylation-sensitive restriction enzymes prior to PCR amplification (Singer-Sam et al., Nucl. Acids Res. 18:687,1990). However, this method has not been shown effective because of a high degree of false positive signals (methylation present) due to inefficient enzyme digestion of overamplification in a subsequent PCR reaction.
Genomic sequencing has been simplified for analysis of DNA methylation patterns and 5-methylcytosine distribution by using bisulfite treatment (Frommer et al., Proc. Natl. Acad. Sci. USA 89:1827-1831, 1992). Bisulfite treatment of DNA distinguishes methylated from unmethylated cytosines, but original bisulfite genomic sequencing requires large-scale sequencing of multiple plasmid clones to determine overall methylation patterns, which prevents this technique from being commercially useful for determining methylation patterns in any type of a routine diagnostic assay.
In addition, other techniques have been reported which utilize bisulfite treatment of DNA as a starting point for methylation analysis. These include methylation-specific PCR (MSP) (Herman et al. Proc. Natl. Acad. Sci. USA 93:9821-9826, 1992); and restriction enzyme digestion of PCR products amplified from bisulfite-converted DNA (Sadri and Hornsby, Nucl. Acids Res. 24:5058-5059, 1996; and Xiong and Laird, Nucl. Acids. Res. 25:2532-2534, 1997).
PCR techniques have been developed for detection of gene mutations (Kuppuswamy et al., Proc. Natl. Acad. Sci. USA 88:1143-1147, 1991) and quantitation of allelic-specific expression (Szabo and Mann, Genes Dev. 9:3097-3108, 1995; and Singer-Sam et al., PCR Methods Appl. 1:160-163, 1992). Such techniques use internal primers, which anneal to a PCR-generated template and terminate immediately 5' of the single nucleotide to be assayed. However an allelic-specific expression technique has not been tried within the context of assaying for DNA methylation patterns.
Therefore, there is a need in the art to develop improved diagnostic assays for early detection of cancer using reliable and reproducible methods for determining DNA methylation patterns that can be performed using familiar procedures suitable for widespread use. This invention was made to address the foregoing need.